1. Field of the Invention
The present invention relates to dielectric filters and dielectric duplexers used in microwave bands and millimeter-wave bands, and the invention also relates to communication apparatuses using the same.
2. Description of the Related Art
FIG. 15A shows a perspective view of a conventional dielectric filter obtained by forming conductor films on a dielectric block and therein. In this figure, reference numeral 1 denotes a substantially rectangular-parallelepiped dielectric block, in which inner-conductor-formed holes 2a and 2b having inner conductors 3a and 3b formed on the inner surfaces thereof are formed. On external surfaces of the dielectric block 1, an outer conductor 4 is each formed. A non-inner-conductor-formed position g is disposed on the inner surface of each of the inner-conductor-formed holes 2a and 2b. In addition, on the external surfaces of the dielectric block 1, input/output electrodes 5a and 5b are formed in such a manner that the electrodes 5a and 5b are isolated from the outer conductors 4.
With this arrangement, TEM-mode dielectric resonators are formed by using the dielectric block, the inner conductors, and the outer conductors. A comb-line coupling between the dielectric resonators is performed by stray capacitances generated at the non-inner-conductor-formed portions g so as to form a dielectric filter constituted of the two-stage resonator as an overall structure.
In the dielectric filter formed by disposing the inner-conductor-formed holes in the substantially rectangular-parallelepiped dielectric block, in order to obtain specified characteristics, the outline dimensions of the dielectric block, the dimensions of the inner-conductor-formed holes, and the positions of the inner-conductor-formed holes disposed inside the dielectric block are determined. Particularly, the unloaded Q of the resonator increases depending on both the thickness tb between the inner-conductor-formed hole and a widthwise line of the dielectric block and the thickness ta between the inner-conductor-formed hole and a lengthwise line of thereof.
FIG. 15B shows the relationship between the ratio of the thickness ta to the thickness tb as the aforementioned two distances and the unloaded Q of the resonator obtained when the length of a widthwise line C of a plane perpendicular to an axis of each of the inner-conductor-formed holes of the dielectric block is set to be 2.0 mm, the length of a lengthwise line H thereof is set to be 4.0 mm, and the axial length D of the inner-conductor-formed hole is set to be 4.0 mm. As shown here, regardless of the width of an inner diameter of each of the inner-conductor-formed hole, the larger the ratio of tb/ta, the higher the unloaded Q of the resonator. In contrast, in a range in which the ratio of tb/ta is larger than 1, almost no increase in the unloaded Q of the resonator can be expected. As a result, in the conventional art, the thickness tb is set to be substantially equal to the thickness ta.
However, in the dielectric filter in which the outer conductor is formed on each of the external surfaces of the substantially rectangular-parallelepiped dielectric block, the arrangement made by the dielectric block and the outer conductors causes a spurious mode such as a TE101 mode, other than a TEM mode as a fundamental resonance mode.
Consequently, such a spurious mode becomes problematic when the spurious mode occurs in a band requiring attenuation, for example, a band of a higher-order frequency than the central frequency of a pass band of the dielectric filter. In a conventional dielectric filter, for example, as shown in Japanese Unexamined Patent Application Publication No. 8-51301, a part of an outer conductor formed on a dielectric-block end face closer to a non-conductor-formed portion is cut away to adjust the resonance frequency of a spurious mode, and, then, the spurious-mode frequency is isolated from a TEM-mode resonance frequency so that influence of the spurious mode can be avoided. As a result, since the overall structure of the dielectric filter is complicated, such an arrangement causes a problem of an increase in production cost.
To overcome the above described problems, preferred embodiments of the present invention provide a dielectric filter and a dielectric duplexer each having a simple structure, in which control can be given in such a manner that no TE-mode spurious response occurs in a band requiring attenuation. Further, the preferred embodiments of the present invention provide a communication apparatus using one of the dielectric filter and the dielectric duplexer.
One preferred embodiment of the present invention provides a dielectric filter and a dielectric duplexer each including a substantially rectangular-parallelepiped dielectric block, a plurality of holes formed inside the dielectric block, the holes aligned in parallel with each other along a lengthwise line of the dielectric block, inner conductors formed on the inner surfaces of the plurality of the holes and outer conductors formed on external surfaces of the dielectric block. In this arrangement, the distance between the central axis of an outermost hole of the aligned holes and a widthwise line of the dielectric block is set to be two times or more than the distance between the central axis of the outermost hole and a lengthwise line thereof.
According to the above described arrangements, since designing of the dielectric block permits the frequency of a spurious mode such as a TE101 mode to be shifted to the lower-frequency side, the influence of the spurious mode can be prevented. As a result, since it is unnecessary to cut away a part of the outer conductor formed on the end face of the dielectric block, production cost can be reduced.
Another preferred embodiment of the present invention provides a communication apparatus including one of the dielectric filter and the dielectric duplexer, which may be used in a high-frequency circuit section relating to signal transmission/reception in the communication apparatus.
According to the above described arrangement, the influence of a spurious mode such as a TE101 mode can be prevented in the dielectric filter and the dielectric duplexer. As a result, limited frequency bands can be effectively used.